Injection Molding
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Continued Dimensional Change of High-Density Polyethylene Injection-Molded Products After Demolding
Problem Phenomenon
The dimensions of the product continue to change within 24-72 hours after demolding, and the final dimensions differ significantly from the dimensions at demolding.
Dimensional instability affects assembly precision and product quality, and is common in crystalline materials.
Cause Analysis
(1) polypropylene has high crystallinity, and continued crystallization after demolding causes post-shrinkage
(2) The demolding temperature is too high; the product has not been fully cooled and continues to shrink at room temperature
(3) The cooling time is insufficient, and the crystallization process is not completed
(4) The environmental temperature changes greatly, and the dimensions of the product fluctuate at different temperatures
(5) The release of internal stress causes slow dimensional changes
Solution
(1) Extend the cooling time to ensure that the demolding temperature of the product is more than 10°C below the heat deflection temperature
(2) Appropriately reduce the mold temperature to the lower value within the range of 40-90°C to ensure full crystallization
(3) Perform annealing treatment on the product after demolding, holding at 30-50°C below the melting point for 2-4h to eliminate internal stress
(4) Control the storage environment temperature to be stable and avoid drastic temperature changes
(5) Establish a dimensional stability inspection system to record dimensional changes at 24h, 48h, and 72h after demolding
Key Takeaway
Post-shrinkage is an inherent characteristic of crystalline materials; dimensional stability is ensured through full cooling, annealing treatment, and environmental control.
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